1. Field of the Invention
The field of invention relates generally to the software arts; and, more specifically, to byte code modification for testing, debugging and/or monitoring of virtual machine based software.
2. Description of the Related Art
1. Bytecode
Certain software technologies, such a Java, emphasize the use of a special interpreter (which may also be referred to as a “virtual machine”) that allows generic processor instructions to be executed on a particular type of processor. Here, each hardware platform (e.g., each computer) that the generic instructions are expected to “run on” typically includes a virtual machine that is responsible for converting the generic processor instructions (which are typically referred to as “bytecode”) into instructions that are specially targeted for the hardware platform's particular processor(s). Software technologies that embrace the execution of bytecode on a virtual machine may be referred to as “virtual machine-based” software.
As a classic instance of the benefit of the Java virtual machine with respect to Internet usage, a first PC that is powered by an Intel processor may download from the Internet the same Java bytecode instructions as a second PC that is powered by a PowerPC processor. Here, the first PC's Java virtual machine converts the Java bytecode into instructions that are specific to an Intel processor while the second PC's Java virtual machine converts the same Java bytecode into instructions that are specific to a PowerPC processor. Thus, through the use of Java bytecode and processor specific Java virtual machines, an Internet server is able to maintain only a single type of code (the Java bytecode) without concern of client compatibility.
FIG. 1a demonstrates the standard compilation flow for generating instructions 105 targeted for a specific processor (“machine specific instructions” 105) from source code 101a, written according to a virtual machine-based language. According to the compilation flow of FIG. 1a, the source code 101a is converted into bytecode 103a by way of a first compiling process 102. The virtual machine then interprets 104 the bytecode into machine-specific instructions 105. The machine-specific instructions are then executed 106 to implement the methods originally articulated by the source code.
2. Object Oriented Programming and Classfiles
Certain software technologies, including Java, are “object oriented.” According to an object oriented approach, the subject matter that is processed by a computer program is organized into classes of likeness. For example, the software used to sell items to customer X might belong to the same class of software (e.g., a class named “sales”) that is used to sell items to customer Y. Here, given that a significant degree of overlap is expected to exist regarding the methods and data types used to process sales for both customers X and Y (e.g., an “update billing about sale” method, an “update accounting about sale” method, a “part number” data type, a “quantity” data type . . . etc.) it is deemed more efficient to organize such methods and data types into a generic “sales” class from which specific instances of the class (e.g., an instance for selling to customer X and an instance for selling to customer Y) can be defined and created.
Each specific instance of a class is referred to as an object; and, each object assumes the characteristics of the class from which it is defined. Thus, a first object could be created from the generic sales class to create a “sell to customer X” object; and, a second object could be created from the generic sales class to create a “sell to customer Y” object. Both objects would be able to execute the methods defined by the class. For example, the “sell to customer X” object could execute the “update accounting about sale” method to update an accounting department about a sale to customer X; and, the “sell to customer Y” object could also execute the “update accounting about sale” method to update an accounting department about a sale to customer Y.
In general, the runtime execution of any computer program can be viewed as the execution of a sequence of methods. With respect to an object-oriented approach, such a sequence of methods can be implemented by calling upon a sequence of objects and invoking one or more methods at each object. In order to invoke the method of an object, a representation of the object is typically created beforehand. In virtual machine-based object-oriented software environments, classfiles are bytecode level data structures from which such representations are created. A classfile can be viewed as a bytecode level description of a class; and, therefore, a classfile can be used as a template for the formation of an object, at the bytecode level, that is a member of the class. As specific methods can be attributed to specific classes, each classfile is made to contain the sequence of bytecode instructions that correspond to each of the class's specific methods.
FIG. 1b shows an embodiment of both a source code level class description 101b and a bytecode level class description (i.e., a classfile) 103b for the exemplary “sales” class referred to above. The source code level class description 101b of FIG. 1b corresponds to level 101a of FIG. 1a; and, the bye code level classfile 103b of FIG. 1b corresponds to level 103a of FIG. 1a. Referring to FIG. 1b, the source code level description of the “sales” class identifies the class's variables 107, such as part number (PN), quantity (QTY), per unit cost ($) and the customer's identity (CUSTOMER), and includes the class's methods such as GetMax 130, UPDATE ACCOUNTING 108, UPDATE BILLING 109, which are articulated as source code software routines.
Compilation 102 of the source code level class description 101b results in the formation of the bytecode level classfile 103b. Among other possible items not shown in FIG. 1b (e.g., rev codes, constant pools, . . . etc), a classfile 103b may contain field information structures 110 for each of the class's variables and method information structures 131, 111, 112 that contain the bytecode level instructions 120b, 113, 112 for each of the class's methods. Both the source code level commands and the bytecode level instructions for the class's methods are illustrated in FIG. 1b as simple lines for illustrative convenience.
Over the course of discussion of various inventive aspects set forth in the detailed description that follows, comparisons will be made against each of FIGS. 1c and 1d. FIG. 1c illustrates, in more detail, exemplary Java source code level commands 120a and corresponding Java bytecode level instructions 120b for the “GetMax” method that was first presented in FIG. 1b. The “GetMax” method is designed to return the greater of two variables ‘a’ and ‘b’ (i.e., if ‘a’ is greater, ‘a’ is returned; if ‘b’ is greater, ‘b’ is returned).
Note that both the source code level and bytecode level implementations for the “GetMax” method have a single entry point 140a, 140b (i.e., the method starts at locations 140a, 140b) and a pair of exit points 141a, 142a and 141b, 142b (i.e., the method can end at locations 141a, 142a and 141b, 142b—noting that a “return” command/instruction causes an output to be presented; which, in turn, can be viewed as the completion of the method). Those of ordinary skill will be able to recognize that: (1) the source code level depiction 120a of the GetMax method observed in FIG. 1c articulates a method written in Java source code language that returns the greater of two values (i.e., a and b); and, likewise, (2) the bytecode level depiction 120b of the GetMax method observed in FIG. 1c articulates a corresponding method written in Java bytecode language that returns the greater of two values (i.e., the values a and b which are respectively loaded on the top of an operand stack by the initial “iload—0” and “iload—1” instructions).
A method having a single entry point 140 and a pair of exit points 141, 142, like the “GetMax” method, can be represented by depiction 150 where the top surface 140 corresponds to an entry point of the method and side edges 141 and 142 correspond to exit points. FIG. 1d shows a schematic representation of the process flow for an exemplary object oriented process. The exemplary process flow of FIG. 1d depicts four methods 205, 206, 207, 208 depicted similarly to the representation 150 observed in FIG. 1c (noting that method_3 207 has a structure that is identical to the structure 150 of FIG. 1c).
Entry points are depicted at the top surfaces 209, 210, 211, 212 of methods 205, 206, 207, 208; and, exit points are depicted as side ports of methods 205, 206, 207, 208 (i.e., side ports 213, 214, 215 for method_1 205; side ports 217, 218 for method_2 206; side ports 220, 225 for method_3 207; and, side ports 221, 222, 223, 224 for method_4 208) and bottom ports of methods 205, 206 and 208 (i.e., bottom port 216 for method_1 205; bottom port 219 for method_2 206; and, bottom port 226 for method_4 208).
According to the object oriented process flow depiction of FIG. 1d, a first object 201 is called in order to execute method_1 205; then, a second object 202 is called in order to execute method_2 206; then, a third object 203 is called in order to execute method_3 207; then, a fourth object 204 is called in order to execute method_4 208. Each of the methods commence at their respective entry points 209, 210, 211, 212. Method_1 exits at a exit point 215, method_2 exits at exit point 217, method_3 exits at exit point 220; and, method_4 exits at exit point 221 endeavors.
3. Enterprise Systems
Traditional client-server systems employ a two-tiered architecture such as that illustrated in FIG. 2a. Applications 231 executed on the client side 230 of the two-tiered architecture are comprised of a monolithic set of program code including a graphical user interface component, presentation logic, business logic and a network interface that enables the client 230 to communicate over a network 233 with one or more servers 234. A database 232 maintained on one of the servers 234 provides non-volatile storage for the data accessed and/or processed by the application 231.
As is known in the art, the “business logic” component of the application represents the core of the application, i.e., the rules governing the underlying business process (or other functionality) provided by the application. The “presentation logic” describes the specific manner in which the results of the business logic are formatted for display on the user interface. The “database” 232 includes data access logic used by the business logic to store and retrieve persistent data.
The limitations of the two-tiered architecture illustrated in FIG. 2a become apparent when employed within a large enterprise. For example, installing and maintaining up-to-date client-side applications on a large number of different clients is a difficult task, even with the aid of automated administration tools. Moreover, a tight coupling of business logic, presentation logic and the user interface logic makes the client-side code very brittle. Changing the client-side user interface of such applications is extremely hard without breaking the business logic, and vice versa. This problem is aggravated by the fact that, in a dynamic enterprise environment, the business logic may be changed frequently in response to changing business rules. Accordingly, the two-tiered architecture is an inefficient solution for enterprise systems.
In response to limitations associated with the two-tiered client-server architecture, a multi-tiered architecture has been developed, as illustrated in FIG. 2b. In the multi-tiered system, the presentation logic 242, business logic 244 and database 246 are logically separated from the user interface 240 of the application. These layers are moved off of the client 230 to one or more dedicated servers on the network 233. For example, the presentation logic 242, the business logic 244, and the database 246 may each be maintained on separate servers, 241, 243 and 245, respectively.
This separation of logic components and the user interface provides a more flexible and scalable architecture compared to that provided by the two-tier model. For example, the separation ensures that all clients 230 share a single implementation of business logic 244. If business rules change, changing the current implementation of business logic 244 to a new version may not require updating any client-side program code. In addition, presentation logic 242 may be provided which generates code for a variety of different user interfaces 240, which may be standard browsers such as Internet Explorer® or Netscape Navigator®.
The multi-tiered architecture illustrated in FIG. 2b may be implemented using a variety of different object-oriented application technologies at each of the layers of the multi-tiered architecture, including those based on the Java 2 Enterprise Edition™ (“J2EE”). In a J2EE environment, the business layer 244, which handles the core business logic of the application, is comprised of Enterprise Java Bean (“EJB”) components with support for EJB containers. Within a J2EE environment, the presentation layer 242 is responsible for generating servlets and Java Server Pages (“JSP”) interpretable by browsers at the user interface layer 240 (e.g., browsers with integrated Java virtual machines).
Although the multi-tiered system illustrated in FIG. 2b provides a more flexible and scalable architecture, it also results in significant additional complexity. For example, monitoring, testing and/or debugging multiple clusters of presentation layer servers, business layer servers and databases, and the dependencies between them requires a significant amount of management overhead. As such, the ability to efficiently monitor, test and/or debug object-oriented, virtual-machine-based enterprise software, such as the software employed in a J2EE environment, is critical for efficient software development and/or implementation.